Laser beam apodization has been a goal of solid state laser programs since the early 1970s. Apodization is the shaping of the spatial beam profile to increase the fill factor through the gain medium. This allows more energy to be extracted from the gain medium and also reduces linear and nonlinear edge diffraction effects which cause self-focusing spikes. A beam apodizer determines, to a large extent, the ultimate performance of a high power laser system.
Several techniques for apodizing laser beams have been reported. See references (1) to (11) listed below. Apodizers based upon absorption are disadvantageous, since the refractive index of the absorbing medium (usually glass or thin films) is changed. The change of refractive index prevents preservation of uniform wavefront quality and can cause Fresnel diffraction effects. Apodization by selective reflection (a distributed Bragg effect) of cholesteric liquid crystals does not give rise to absorption as in absorbing media. Recently, S. D. Jacobs et al. have developed a new apodizer based on liquid crystal technology that has demonstrated properties resembling those of a perfect apodizer. See reference (12) and Jacobs et al. U.S. Pat. No. 4,679,911. The Jacobs et al. apodizer works best for small clear apertures, e.g. up to about 8 mm., and in some embodiments requires grinding of precise flat surfaces of optical elements, which makes fabrication difficult.
It is a feature of the invention to provide a laser beam apodizer utilizing liquid crystals which can be used for beam apodization of an oblong (in cross-section) beam, especially suitable for use with slab-geometry laser amplifiers or diode lasers, and also for beam apodization of circular beams, especially useful with rod laser amplifiers with large (e.g., 10 mm or more) clear apertures.
The invention is especially suitable for use in providing a soft aperture for shaping the intensity of laser beams utilizing the properties of cholesteric liquid crystals (CLC). The term "liquid crystals" as used herein includes crystals both in the solid and fluid state. The term "cholesteric" is used generically to mean liquid crystals which have chirality, whether pure cholesteric compounds or nematic materials mixed with chiral additives.
CLCs have a helical layered organization as shown in FIG. 1. Normally, a CLC cell is prepared between two glass substrates. Within each single layer of the structure, molecules align in a parallel configuration like nematics. The average orientation of the elongated liquid crystal molecules is defined as the director. In adjacent layers, protruding side groups of atoms attached to each molecule and force the director to be twisted. The rotation of layers through the fluid gives rise to a helical structure. One full 360.degree. rotation of the director is defined as one pitch length P.sub.o. This helical structure leads to the important optical properties of selective reflection in circular polarization and wavelength. See reference (13).
Consider a right-handed CLC cell shown in FIG. 1 whose helical axis is oriented along the z-axis. No interaction occurs between the right-handed helical structure of the CLC and left circularly polarized light propagating through it. However, when right circularly polarized light with wavelength .lambda. propagates along the z-axis at normal incidence, the reflectivity R is given by ##EQU1## where ##EQU2## is the coupling coefficient; ##EQU3## is the detuning parameter; .lambda..sub.o =n.sub.av .multidot.P.sub.o is the peak wavelength of the selective reflection band; and L is the CLC fluid thickness. See reference (14). .DELTA.n=(n.sub.e -n.sub.o) and n.sub.av =(n.sub.e +n.sub.o)/2 represent the optical birefringence and average refractive index, respectively. When .lambda.=.lambda..sub.o, the CLC structure is well phase-matched to the input wavelength, and the reflectivity is equal to EQU R=tanh.sup.2 (.kappa..sub.o L). (2)
It has been reported see reference (1)) that an ideal beam apodizer possesses the following characteristics:
1. The slope of the transmission function between 90% and 10% transmission points is at least 3 .lambda.L/D , where .lambda. is the laser wavelength, L is the propagation distance over which intensity modulation should be minimal, and D is the beam diameter. One of the transmission functions which approximates this condition is a super-Gaussian of order N, that is, EQU T(r)=exp[-(r/r.sub.o).sup.N ] (3) PA0 2. Wave front quality over the clear aperture and into the soft edge is a smooth function with continuous first derivatives. PA0 3. The peak to minimum transmission ratio is at least 1000 to 1. PA0 4. High laser damage threshold at the design wavelength and pulse width. PA0 5. Environmental stability.
where the radial clear aperture r.sub.o is selected based on the condition that T(r.sub.1)=10.sup.-3, where 2r.sub.1 is the entry aperture of the optical device (e.g. a laser amplifier) that follows the apodizer. Transmission is Gaussian where N=2.